1. Field of the Invention
The present invention relates to a recording medium such as an optical disk for recording digital data as pits in high density, a recording apparatus for recording digital data on the recording medium, and a reproduction apparatus for reproducing the digital data recorded in the recording medium.
2. Description of the Related Art
A SCIPER (Single Carrier Independent Pit Edge Recording) system is known as a method for recording digital data in a recording medium such as an optical disk in high density.
In the SCIPER system, pits are arranged at a predetermined interval along the circumferential direction on the tracks of an optical disk, and the positions of the front edge and the rear edge of each pit are changed stepwise, respectively, in accordance with the digital data to be recorded. Namely, in the SCIPER system, the position of the pit itself is not changed but the positions of the front edge and the rear edge of the pit are changed, respectively, for recording digital data on the tracks. The front edge indicates the edge located at the front portion of the pit, and the front portion of the pit indicates the portion located in one of the circumferential directions of the optical disk. The rear edge indicates the edge located at the rear portion of the pit, and the rear portion of the pit indicates the portion located in the other circumferential direction of the optical disk.
FIG. 7 shows pits formed on an optical disk according to the SCIPER system. In FIG. 7, assume that the beam spot moves in the direction indicated by arrow a on the optical disk. The positions designated by reference numerals FE1 to FE3 in the pit PT constitute the front edge, and the positions indicated by reference numerals RE1 to RE3 constitute the rear edge. In FIG. 7, solid lines indicate the contour of the pits PT which are actually formed on the optical disk, and dotted lines indicate the other contour of the pits which are formed when the other recording symbols are assigned to these pits.
The pits PT shown in FIG. 7 are arranged at a predetermined interval D3. The front edge of each pit PT represents a recording symbol having one of ternary levels of xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c+1xe2x80x9d by changing the position thereof in three steps. The rear edge of each pit PT also represents a recording symbol having one of ternary levels of xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c+1xe2x80x9d by changing the position thereof in three steps.
When the recording symbol is xe2x80x9cxe2x88x921xe2x80x9d, for example, the front edge takes a position indicated by reference numeral FE3 and thereby the length of the pit PT is shortened. When the recording symbol is xe2x80x9c0xe2x80x9d, on the other hand, the front edge takes a position indicated by reference numeral FE2, which is an intermediate position of the three positions. When the recording symbols is xe2x80x9c+1xe2x80x9d, the front edge takes a position indicated by reference numeral FE1 and thereby the length of the pit PT is lengthened.
In similar fashion, when the recording symbol is xe2x80x9cxe2x88x921xe2x80x9d, the rear edge takes a position indicated by reference numeral RE1 and thereby the length of the pit PT is shortened. When the recording symbol is xe2x80x9c0xe2x80x9d, the rear edge takes a position indicated by reference numeral RE2, which is an intermediate position of the three positions. When the recording symbol is xe2x80x9c+1xe2x80x9d, the rear edge takes a position indicated by reference numeral RE3 and thereby the length of the pit PT is lengthened.
When reproducing the digital data from an optical disk, the reproduction apparatus radiates a reproducing laser beam toward the optical disk thereby to form a beam spot SP on the pits PT as shown in FIG. 7. At the same time, the reproduction apparatus moves the beam spot SP in the direction of arrow a in FIG. 7. In this way, the reproduction apparatus detects an analog detection signal corresponding to the recording symbol recorded as a position of the front edge or the rear edge. As a result, the level of the analog detection signal is changed in three steps in accordance with the levels xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c+1xe2x80x9d of the recording symbol corresponding to the position of the front edge or the rear edge. The reproduction apparatus performs the filter operation for emphasizing the high frequency band of the analog detection signal, detects the levels of the three steps and reproduces the digital data.
Next, the level detection of the analog detection signal will be explained with reference to FIG. 8. FIG. 8 is a diagram showing waveforms of the analog detection signal. In FIG. 8, a multiplicity of signals that can be output as an analog detection signal are shown in overlapped form.
The reproduction apparatus sample-holds the analog detection signal at the timing indicated by T1, T2, T3 and T4 in FIG. 8. The reproduction apparatus then compares the level obtained by sample-holding with preset threshold values, and determines which of xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c+1xe2x80x9d is indicated by the level.
The reproduction apparatus compares the level obtained by sample-holding with two threshold values xe2x80x9cxe2x88x920.5xe2x80x9d and xe2x80x9c+0.5xe2x80x9d. If the level obtained by sample-holding is smaller than xe2x88x920.5, the reproduction apparatus determines that the particular level represents xe2x80x9cxe2x88x921xe2x80x9d. If the level obtained by sample-holding is not smaller than xe2x88x920.5 but not larger than 0.5, the reproduction apparatus determines that the particular level represents xe2x80x9c0xe2x80x9d. If the level obtained by sample-holding is larger than +0.5, the reproduction apparatus determines that the level is xe2x80x9c+1xe2x80x9d.
The technique of the SCIPER system is described, for example, in Japanese Laid-open Publication No. Hei 6-76303.
On the other hand, a research is under way for the technique to make possible the recording and reproducing operation of higher density by combining the SCIPER system with the conventional RPR (Radial Direction Partial Response) system. The recording and reproducing operation by a combination of the SCIPER system and the RPR reproduction system is described, for example, in xe2x80x9cPartial Response Recording in Radial Directionxe2x80x9d, 1997 Optical Data Storage Conference Digest, pp. 42-43, (Apr. 1997).
Next, a pit arrangement of the optical disk and the reproduction of the digital data from the optical disk by the recording and reproduction technique of the combined SCIPER system and the RPR reproduction system will be explained with reference to FIGS. 9A and 9B. FIG. 9A is a plan view showing a pit arrangement of the optical disk according to the conventional recording and reproduction technique using the combined system of SCIPER and RPR, and FIG. 9B is a diagram showing waveforms of the analog detection signal. In FIG. 9B, a multiplicity of signals that can be output as an analog detection signal are all shown in overlapped form. However, only one of the multiplicity of waveforms shown in FIG. 9B is output as the analog detection signal. Further, the description below refers to the case in which the positions of the front and rear edges of each pit correspond to the recording symbols having one of ternary levels of xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c+1xe2x80x9d. Also, in order to indicate that the positions of the front and rear edges of each pit are changed in three steps in accordance with the recording symbol, all the three positions that each edge can take are indicated by solid lines in FIG. 9A. The actual edge position of course is one of the three positions. This is also the case with FIG. 10A described later.
In the case where digital data are reproduced by the recording and reproduction technique combining the SCIPER system and the RPR reproduction system, the beam spot is arranged with the center thereof coincident with the center line CL1 between adjacent two tracks T, and moved along this center line CL1. Thus, adjacent two tracks T are radiated at the same time by the laser beam.
Further, in the recording and reproduction technique combining the SCIPER system and the RPR reproduction system, the pit trains are in phase with each other between adjacent tracks T. Namely, the pits PT are arranged radially of the optical disk, a pit PT on one track T adjoins another pit on another track T, and one land LD adjoins another land LD, as shown in FIG. 9A. The land LD indicates an area between two adjacent pits PT in the circumferential direction, i.e. the area where no pit PT is formed.
Further, when reproducing the digital data by the recording and reproduction technique combining the SCIPER system and RPR reproduction system described above, the reproduction apparatus sample-holds the level of the analog detection signals detected at the timing when the front edges of the radially adjacent two pits PT are irradiated at the same time or the timing when the rear edges of the same two pits are irradiated at the same time.
Assume that the level of the analog detection signal decreases when the beam spot SP is located on pits. ST1 and ST2 in FIG. 9B indicate the time points when the beam spot comes to the positions SP1 and SP2 in FIG. 9A respectively. At these time points, the reproduction apparatus sample-holds the levels of the analog detection signal.
In the example shown in FIGS. 9A and 9B, the pits PT formed on the two tracks T are detected at the same time, and each PT has an edge at the position changed in accordance with the recording symbol having one of the ternary levels xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c+1xe2x80x9d. The level obtained by sample-holding the analog detection signal, therefore, indicates one of the sums of two recording symbols. Namely, the level obtained by sample-holding indicates one of the five levels xe2x80x9cxe2x88x922xe2x80x9d, xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c+1xe2x80x9d, xe2x80x9c+2xe2x80x9d. The reproduction apparatus compares the level obtained by sample-holding with predetermined threshold values and thus specifies one of the five values thereby to reproduce the digital data.
Further, as shown in FIG. 9B, the analog detection signal alternates between rise and fall at each sample timing. This is because the pit trains are in phase with each other between adjacent tracks T.
Namely, during the period when the front edge of the pit PT is included in the beam spot, the area of the pit PT included in the beam spot increases and therefore the amount of light reflected from the optical disk decreases with time. During the period when the rear edge of the pit PT is included in the beam spot, on the other hand, the area of the pit PT included in the beam spot decreases and therefore the amount of light reflected from the optical disk increases with time. As a result, the waveform of the detection signal is inclined at each sample timing.
On the other hand, a research is under way on the technique of combining the SCIPER system with what is called the two-dimensional PRML (Partial Response Maximum Likelihood) according to the prior art. The recording and reproduction technique based on the combination of the SCIPER system and the two-dimensional PRML reproduction system is described, for example, in xe2x80x9cSimulation of a High Density Optical Disk System Employing Multi-Level Pit Edge Recording and 2 Dimensional PRML Reproductionxe2x80x9d, Oct. 1998, Research Report MR98-30, Magnetic Recording Research Committee, the Institute of Electronics, Information and Communication Engineers.
Next, the pit arrangement of an optical disk and the reproduction of digital data from the optical disk by the recording and reproduction technique using the combination of the SCIPER system and the two-dimensional PRML reproduction system will be explained with reference to FIGS. 10A and 10B. FIG. 10A is a plan view showing the pit arrangement of the optical disk according to the conventional recording and reproduction technique using a combination of the SCIPER system and the two-dimensional PRML reproduction system, and FIG. 10B is a diagram showing waveforms of the analog detection signal. A multiplicity of signals likely to be output as detection signals are shown in overlapped form in FIG. 10B. Further, the description below refers to the case in which the positions of the front and rear edges of each pit correspond to the recording symbols having one of the ternary levels xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c+1xe2x80x9d.
When the digital data are reproduced by the recording and reproduction technique combining the SCIPER system and the two-dimensional PRML reproduction system, the beam spot is arranged with the center thereof coincident with the center line CL2 between adjacent two tracks T and moves along the center line CL2. The adjacent two tracks T are irradiated at the same time by the laser beam.
Further, in the recording and reproduction technique combining the SCIPER system and the two-dimensional PRML reproduction system, like the recording and reproduction technique combining the SCIPER system and the RPR reproduction system, the pit trains are in phase with each other between adjacent tracks T.
Furthermore, when reproducing digital data by the recording and reproduction technique combining the SCIPER system and the two-dimensional PRML reproduction system, the reproduction apparatus sample-holds the levels of the analog detection signal detected (i) at a timing when the front edge and the rear edge of two radially adjacent pits PT, respectively, are irradiated at the same time, or (ii) at a timing when the rear edges of adjacent two pits in the radial direction and the front edges of adjacent two pits PT in the circumferential direction are irradiated at the same time.
Assume that the level of the analog detection signal decreases when the beam spot SP is located on pits. ST3 and ST4 in FIG. 10B indicate the time points when the beam spot comes to the positions SP3 and SP4 in FIG. 10A respectively. At these time points, the reproduction apparatus sample-holds the levels of the analog detection signal.
In the example shown in FIGS. 10A and 10B, a total of four edges of the pits PT formed on the two tracks T are detected at the same time, and each edge of the pits changes its position in accordance with the recording symbol having one of the ternary levels xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c+1xe2x80x9d. As a result, the level obtained by sample-holding the analog detection signal indicates one of the sums of the four recording symbols. Namely, the level obtained by sample-holding indicates one of the nine values xe2x80x9cxe2x88x924xe2x80x9d to xe2x80x9c+4xe2x80x9d. The reproduction apparatus compares the level obtained by sample-holding with predetermined threshold values and by thus specifying one of the nine values, reproduces the digital data.
Further, as shown in FIG. 10B, the level of the analog detection signal varies at each sample timing. This is because pit trains are in phase with each other between adjacent tracks T, as described above.
Namely, when the beam spot is located at the position indicated by SP3 in FIG. 10A, the pits PT occupy considerable area in the beam spot range, and therefore the amount of the light reflected from the optical disk is small. When the beam spot is located at the position indicated by SP4 in FIG. 10A, on the other hand, the pits PT occupy small area in the beam spot range, and therefore the amount of the light reflected from the optical disk is considerable. As a result, the level of the detection signal is varied from one sample timing to another.
As mentioned earlier, in the recording and reproduction technique combining the SCIPER system and the RPR reproduction system described with reference to FIG. 9, the pit trains are in phase with each other between adjacent tracks T. Therefore, the waveform of the reproduction signal is inclined at each sample timing. Further, in the case where the reproduction clock contains a timing offset or a jitter, the level of the sample value obtained by sampling considerably fluctuates so that accurate data reproduction is difficult by the comparison with the threshold values (indicated by the lateral dashed lines in FIG. 9B), resulting in a deteriorated reproduction performance.
Also in the recording and reproduction technique combining the SCIPER system and the two-dimensional PRML reproduction system described with reference to FIGS. 10A and 10B, the pit trains are in phase with each other between adjacent tracks T. Therefore, the center level of the detection signal is varied from one sample timing to another. As a result, in the case where the digital data are decoded by the Viterbi decoding method from the sample values, for example, two groups of nine prediction values (first prediction value group and second prediction value group in FIG. 10B) should be provided to meet the variations of the center level, which groups are required to be switched alternately at each sample timing. This leads to the problem of a complicated information reproduction apparatus. Namely, in FIG. 10B, when the overall level of the first prediction value group used at the timing ST3 is compared with the overall level of the second prediction value group used at the timing ST4, it is seen that the level of the second prediction value group is higher than that of the first prediction value group.
As generally known as Hopkins"" optical theorem, on the other hand, the amount of the light reflected from embossed pits is not proportional to the pit area and is a nonlinear response. Thus, in the recording and reproduction technique combining the SCIPER system and the two-dimensional PRML reproduction system, the detection signal fails to take a simple level representing the sum of four data (nine levels xe2x80x9cxe2x88x924xe2x80x9d to xe2x80x9c4xe2x80x9d in the case of FIG. 10B) but shows a nonlinear level distribution. This nonlinear distortion of the detection signal becomes more conspicuous with the change of the pit area in the beam spot SP. In the conventional combination of the SCIPER system and the two-dimensional PRML reproduction system, the pit area in the beam spot SP considerably varies with each sample timing, resulting in a large nonlinear distortion of the detection signal. Thus, the dispersion is increased in the level distribution of the sample values, thereby leading to the problem that accurate data reproduction becomes more difficult and the reproduction performance is deteriorated.
An object of the present invention is to provide a recording medium, a recording apparatus and a reproduction apparatus capable of preventing the deterioration of the reproduction performance without complicating the configuration of the information reproduction apparatus even in the case where a high-density recording and reproduction operation is performed using the various recording and reproduction techniques described above.
A recording medium in accordance with the present invention includes a substrate member, a plurality of recording tracks formed on the substrate member, and a plurality of pits formed on the recording tracks. In the recording medium, digital information is recorded as pits. The pits are arranged in a fixed period. The position of an edge of each of the pits is changed in accordance with the digital information. The phase of the train of the pits formed on one of the recording tracks and the phase of the train of the pits formed on another one of the recording tracks adjacent to the one of the recording tracks are shifted each other by half of the fixed period.
When the digital information is reproduced from the recording medium according to the RPR system, PRML system or the like, at least two recording tracks adjacent to each other are simultaneously irradiated with a light beam, and the positions of the edges of at least two pits formed on the two recording tracks are simultaneously detected. The detection of the pits is achieved by receiving the light beam reflected by the recording tracks and generating a detection signal representing, for example, the amount of the reflected light. If the ratio of the total areas of the pits to the whole area that is irradiated with the light beam is changed, the amount of the reflected light is changed. Therefore, the level of the detection signal is also changed.
In the aforementioned conventional recording medium, the phase of the train of the pits formed on the recording track is identical with the phase of the train of the pits formed on the adjacent recording track. Because of this, the ratio of the total areas of the pits to the whole areas that is irradiated with the light beam is greatly changed with the movement of the detecting position (the position of the light spot of the light beam).
In the recording medium in accordance with the present invention, on the other hand, the phase of the train of the pits formed on the recording track and the phase of the train of the pits formed on the adjacent recording track are shifted each other by half of the fixed period. Therefore, the change of the ratio of the total areas of the pits to the whole areas that is irradiated with the light beam, with the movement of the detecting position, can be reduced. As a result, the center level of the detection signal can be made substantially constant. Thus, the determination of the edge positions of the pits can be made easier and more accurate, and the performance of the reproduction of the digital information can be improved.
In the recording medium, a disk may be used as the substrate member. In this case, the recording tracks are extended in the circumferential direction of the disk, and the position of the edge of each of the pits is changed in the circumferential direction of the disk.
In the recording medium, the position of the edge of each of the pits may be changed in three steps in accordance with the digital information.
In the recording medium, a digital information recording area and a control information recording area may be formed on the substance member. In this case, the pits corresponding to the digital information are recorded in the digital information recording area, and a plurality of control pits corresponding to control information are recorded in the control information recording area. The control information is information for a detection of the position of the edge of each of the pits corresponding to the digital information.
An information reproducing apparatus in accordance with the present invention is an apparatus for reproducing digital information recorded in the recording medium in which the phase of the train of the pits formed on a certain recording track and the phase of the train of the pits formed on the adjacent recording track are shifted each other as mentioned above. The information reproducing apparatus includes: a detecting device that simultaneously irradiates at least two of the recording tracks adjacent to each other with a light beam, simultaneously detects the positions of the edges of at least two of the pits formed on the at least two of the recording tracks, based on the light beam reflected by the at least two of the recording tracks, and generates a detection signal indicating the detected positions of the edges; and a reproducing device that reproduces digital information, based on the detection signal. In reproducing the digital information, the center level of the detection signal can be made substantially constant. Thus, the determination of the edge positions of the pits can be made easier and more accurate, and the performance of the reproduction of the digital information can be improved without complicating the structure of the information reproducing apparatus.
In the information reproducing apparatus, the reproducing device may include a Viterbi decoding device that decodes the detection signal in accordance with a Viterbi decoding method. By using the Viterbi decoding method, the accuracy of the reproduction of the digital information can be improved.
An information recording apparatus in accordance with the present invention is an apparatus for recording digital information in a recording medium in which the phase of the train of the pits formed on a certain recording track and the phase of the train of the pits formed on the adjacent recording track are shifted each other as mentioned above.
The information recording apparatus includes: a first generating device that generates a first pulse signal having a fixed period, whose rising timing and falling timing are changed in accordance with the digital information; a second generating device that generates a second pulse signal having the fixed period, whose rising timing and falling timing are changed in accordance with the digital information, and whose phase is shifted by 180 degrees from a phase of the first pulse signal; and a pit forming device that forms a plurality of pits on the recording tracks by using the first pulse signal and the second pulse signal, thereby recording the digital information on the recording tracks as the pits. The pit forming device forms a first part of the pits on one of the recording tracks such that positions of edges of each of the pits in the first part are changed in accordance with the rising timing and the falling timing of the first pulse signal. The pit forming device forms a second part of the pits on another one of the recording tracks adjacent to the one of the recording tracks such that positions of edges of each of the pits in the second part are changed in accordance with the rising timing and the falling timing of the second pulse signal.
Since the first pulse signal is 180 degrees out of phase with the second pulse signal, the phase of the pit train formed by using the first pulse and the phase of the pit train formed by using the second pulse are shifted each other by 180 degrees. The pit forming device alternately uses the first pulse signal and the second pulse signal, and switches them over each time the formation of the pits on one recording track is finished. As a result, the phase of the pit train on the recording medium is reversed for each recording track.
If the substrate member of the recording medium is a disk and the recording tracks are extended in a circumferential direction of the disk, then the pit forming device forms the pits such that the positions of the edges of each of the pits are changed in the circumferential direction of the disk.
In the information recording apparatus, the pit forming device may form the pits such that the positions of the edges of each of the pits are changed in three steps in accordance with the digital information.
The nature, utility, and further feature of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.